Background reduction in image tube



INVENTORS Richard L.Longini uglRolph QMclntosh.

v ATTORNEY Dec. 13, 1955 R. L. LONGIINI EIAL BAGKGROUND REDUCTION INIMAGE TUBE Filed Jan. 22, 1952 WITNESSES: 1,. a wj/ha United StatesPatent BACKGROUND REDUCTION IN IMAGE TUBE Richard L. Longini and Ralph0. McIntosh, Forest Hills, Pa., assignors to Westinghouse ElectricCorporation, East Pittsburgh, Pa., a corporation of Pennsylvania 7Application January 22, 1952, Serial No. 267,598

11 Claims. (Cl. 313-101) Our invention relates to electrical dischargedevices and in particular covers an arrangement for minimizingbackground light in electrical discharge tubes embodying fluorescentscreens. Certain features of our invention are, however, applicable toprevention of undesired ionization in high-vacuum electrical dischargetubes of other types. The invention is particularly useful in imageintensifier tubes in which electron-images formed by photoelectricscreens are accelerated into impact with electron-phosphor screens toproduce light images as exemplified in Mason and Coltman U. S. Patent2,523,132 issued September 19, 1950.

A tube found useful in the image intensifier described in theabove-mentioned patent is shown schematically in Fig. 1 and comprises ahigh-vacuum glass container 1 having an electron-phosphor output screen2, of zinccadmium-sulphide, at one end on which a light-image isproduced by impact of electrons which emanate as an electron-image froma photo-electric layer 4 on the input screen 3 at the other end ofcontainer 1. The input screen 3 comprises also a layer 5 of fluorescentmaterial supported closely adjacent the photoelectric layer, and alight-image is generated in the layer 5 by X-rays projected onto itthrough an observed object outside the container. The light-image inlayer 5 reacts on photoelectric layer 4 to generate an electron image atits surface, and suitable positive potentials impressed by means notshown on the output screen 2, and on an electron-lens system comprisingmetal collars 6, 7, 8, accelerate the electron-image into incidence onoutputscreen 2 to generate a light image of high brightness there. Anintensified replica of the light-image of input screen 3 is thusproduced on output screen 2.

We have in practice found that there is present on the output screen 2,in addition to the desired lightimage produced by the impact thereon ofthe electronimage generated at photoelectric layer 4, what may be termeda background glow, which exists even when no X-rays are acting on thefluorescent input-layer 5. This background glow is more or lessuniformly distributed over output screen 2, but it seriously impairs thecontrasts and general visibility of the desired replica of the X-rayimage generated on input screen 3, particularly where the latter is oflow intensity. We have found that this background glow arises largelyfrom electrons arising from high electric field emission at electrodesof the tube, or from other causes, at places where the electricalpotential gradient is high. These electrons are accelerated by theelectric field, some of them directly into impact on theelectron-phosphor output screen 2, and others into impact upon otherparts of the tube at which they are reflected, to the screen 2, orgenerate secondary electrons which find their way to screen 2. All suchimpacting electrons generate background glow on screen 2. Many other ofthese unwanted electrons strike the glass tube walls and insulationinside the tube, both of which are somewhat fluorescent and produce abackground glow which irradiates the photoelectric layer 2,727,173Patented Dec. 13, 1955 4 and generates more unwanted electrons whichgenerate further background glow at output screen 2.

It is accordingly necessary to prevent as far as possible the emissionof electrons at and around the electrical components within container 1.

One object of our invention is accordingly to minimize the production ofelectrons by electrical gradients within electrical discharge tubeswhich contain fluorescent materials.

Another object is to minimize the production of electrons by electricalgradients within electrical discharge tubes which contain photoelectricmaterials.

Still another object is to minimize the production of electrons byelectrical gradients within electrical discharge devices which containboth fluorescent materials and photoelectric materials.

Another object is to minimize background glow in devices in whichelectron images are projected by electric fields into incidence onfluorescent screens.

Another object is to minimize the production of stray fluorescent lightin devices in which photoelectric materials are present in company withinsulating materials and conductors of high electrical potential.

A further object is to avoid the production of substantial backgroundglow on the output screens of image intensifier tubes.

Other objects of our invention will become apparent upon reading thefollowing description taken in connection with the drawings, in which:

Figure 1 is a schematic showing of an image intensifier tube to whichour invention is particularly applicable;

Fig. 2 is a detail view of a conductor provided with one embodiment ofour invention; and

Fig. 3 is a detail view of a conductor provided with another embodimentof our invention.

The general structure of Fig. 1 has already been described, but itsdetails will be explained as follows: The input screen 3 and the metalcollars 6, 7 and 8 are supplied with proper electrical potentials byleads 11 and 13 sealed through container 1 and the connections withincontainer 1 are, as a matter of tube manufacture, conveniently made byrods or wires 14 of rather small diameter; i. e., a diameter under threemillimeters. Since certain of these rods have potentials many thousandvolts different from other elements, lines of electric force reach highconcentrations at the surfaces of some of the above-mentioned rods andso set free electrons from the electrode metal. It has been found thatthe primary sources of unwanted electrons are the conducting elements ofthe tube having small radii of curvature where large potentialdifferences are present.

' We have found'that background glow on the output screen 2 ispractically eliminated by covering all rods and wires within theenclosure 1 with metallic or other conducting beads such as 15 in Fig.2. Such beads are shaped so as to avoid all edges and points, and tohave no portions of their surface of small radius of curvature. Theirsurfaces should be smooth. Specifically no radius of curvature should beless than 2 mm. It will be noted that the rods or wires may be bent andcurved, yet a series of standard beads strung on them will successfullyprevent difliculty from background glow at the output screen. Theability to use standard beads which can be produced cheaply in largequantities and combined with wires or rods cut from reels and producedby similarly cheap commercial methods to form electrode structure hasenormous practical advantages in tube manufacture compared with thesituation which would exist it special conductors of the required sizeand shape had to be manufactured in integral form. Our invention notonly solves the problem of minimizing background light but does so in away which is highly economical and simple in large-scale production.Furthermore, it solves the problem not only for image intensifier tubes,but for other types of apparatus where photoelectric materials orfluorescent materials are present and subject to electric force.

Fig. 3 shows another form of bead which may be employed to carry out theprinciples of our invention. The beads 16 has the form of a cylinderwith the peripheral edges of the end faces rounded, thereby avoidingintense electric field concentration and electron emission which wouldoccur were the edge left sharp. Complete shielding of the underlyingconductor is also afforded, where the latter is bent into a curve.

We claim as our invention:

1. In combination, in an evacuated vacuum-tight enclosure, a surface offluorescent material, means to impress a high electrical potentialdifference in said enclosure between a conductor with a cross-sectionhaving a small radius of curvature and another point, and electricallyconducting beads having surfaces devoid of portions having small radiusof curvature strung on said conductor.

2. In combination in a highly-evacuated vacuum-tight enclosure, asurface of photoelectric material, means to produce a high electricalpotential difference in said enclosure between a conductor with across-section having a small radius of curvature and another point, andelectrically conducting beads having surfaces devoid of portions havinga radius of curvature less than 2 times the aforesaid radius ofcurvature strung on said conductor.

3. In combination, in a highly-evacuated vacuum-tight enclosure, asurface of photoelectric material, means to produce a high electricalpotential difference in said enclosure between a wire of not over onemillimeter diameter and another point, and electrically conducting beadswith surfaces having no radius of curvature less than 2 millimetersstrung on said: wire.

4. in combination, in a highly-evacuated vacuum-tight enclosure, asurface of fluorescent material, means to produce a high electricalpotential difference in said enclosure between a wire of not over 3millimeters diameter and another point, and electrically conductingbeads with surfaces having no radius of curvature less than 2millimeters strung on said wire.

5. In combination, in a highly-evacuated vacuum-tight enclosure having aglass wall, a surface of fluorescent material, means to produce a highelectrical potential difference in said enclosure between a wire notover 3 milimeters in diameter and another point, and electricallyconducting beads with surfaces having no radius of curvature less than 2millimeters strung on said wire.

6. In combination, in a highly-evacuated vacuum-tight enclosure having aglass wall, a surface of photoelectric material, means to produce a highelectrical potential difference in said enclosure between a wire of notover 3 millimeters diameter and another point, and electricallyconducting beads with surfaces having no radius of curvature less than 2millimeters: strung on said wire.

7. In combination, in a highly-evacuated vacuum-tight enclosure having aglass wall, a surface of photoelectric material, means to produce a highelectrical potential difference in said enclosure between a conductorwith a surface having a small radius of curvature and another point, andelectrically conducting beads having no radius of curvatureless than 2'times'the aforesaid radius of curvature strung on said conductor.

8. In combination, in a highly-evacuated vacuum-tight enclosure having aglass, wall, a surface of fluorescent material, means to produce a, highelectrical potential difference in said enclosure between a conductorwith a surface having a small radius of curvature and another point, andelectrically conducting beads having no radius of curvaturev less than 2times the aforesaid radius of curvature strung on said conductor.

9. In combinatiomin a highly-evacuated vacuum-tight enclosure containinga surface of insulating material, a surface of fluorescent material,means to produce a high electrical potential difference in saidenclosure between a conductor with a surface having a small radius ofcurvature and another point and, electrically conducting beads havingsurfaces with no radius of curvature less than 2 times the aforesaidradius of curvature strung on said conductor.

10. In combination, in a highly-evacuated vacuumtight enclosurecontaining a surface of insulating material, a surface photoelectricmaterial, means to produce a high electrical potential difference insaid enclosure between a conductor with a surface having a small radiusof curvature and another point and electrically conducting beadshavingsurfaces with no radius of curvature less than 2 times the aforesaidradius of curvature strung on said conductor.

11. In an evacuated electron tube, a screen of zinceadmium-sulphide, asource of electrons, leads sealed through said tube walls for impressinga high electrical potential difference between said screen and saidsource, connecting wires within said tube of diameter not over onemillimeter connected to said leads and impressed with said potentialdifference, and electrically conducting beads having no exposed surfaceof less than 2 millimeters radius of curvature strung onsaidwires.

References Cited in the file of this patent UNITED STATES PATENTS450,734 Bunker Apr. 21, 1891 1,171,936 Faccioli Feb. 15, 1916 2,223,198Zittrick et a1. Nov. 26, 1940 2,452,620 Weimer Nov. 2, 1948 2,523,132Mason et all Sept. 19, 1950

